Friday, December 12, 2014

October 2018


   Since the 1970’s researchers have participated in a global effort to find sustainable energy sources for motor vehicles and electric power generation. This has led to an increased need for clean and reliable energy storage devices, which can store the power generated from clean energy sources, and make it readily available when needed in a wide range of applications. Energy storage has in general been dominated by electro-chemical batteries, which are hazardous, resource-intensive to manufacture, and have limited number of charge cycles. Potential now exists, with the growing viability of synthetic fuel gasification [1], to advance deployment of phase change (“source + sink”) storage, in which liquefied syngas with dissolved hydrogen vaporizes during heat sink cooling before firing the cryo-compression gas turbine (Cryo-GT) prime mover [2]. The cryo-sink reduces gas turbine fuel consumption by one–half and by two-thirds as compared to a normally aspirated Brayton and Otto Cycle engine, respectively. Small gas turbine application is enabled, while reducing compressor/turbine work and pressure ratio.Phase change storage previously relied on intermittent off-peak grid, solar and wind to provide liquefied refrigerant. In advanced phase change storage, wood and coal gasifiers operate with carbon neutral combustion to provide integral synthesis of cryo-fuel (heat source) and refrigerant (heat sink). The term “source + sink storage” refers to recovery of gasifier heat to split water in a thermo-chemical reactor [3] for hydrogen enrichment of syngas, while let-down of gasifier fuel pressure drives downstream fuel and air liquefiers. This “third heating way” is potentially a more economical high temperature primary source than the two alternatives; helio/receiver solar and nuclear sources, currently under development. Both of these primary heat sources are unproven, inefficient and expensive with difficult high temperature design requirements. Oxygen by-product of the thermo-chemical reactor may enrich combustion air to the gasifier or be liquefied for other use.
   A “liquid nitrogen economy” was proposed in 1974 [4] and some high pressure engines with cryogenic compression were built and tested, including a fired turbine [5] and two fuel-less reciprocating engines [6, 7]. Subsequently, liquid nitrogen storage began gaining acceptance as indicated by an operating 300 kW pilot plant and development of a peaking turbine and a liquid air vehicle engine [8]. The Cryo-GT has a unique cryo-compression system. It is well suited for smaller low pressure motor vehicle application and for distributed generation in an integrated gasification combined cycle (IGCC) [9].

Advantages of phase change storage over battery storage include:
* long service life with no disposal requirement,
* consistent efficient performance,
* universal availability,
* low weight and capital cost in a well developed technology, and
* less hazardous in terms of toxicity and high temperature fire safety.

Additional features enhancing fuel and sink refrigerant synthesis include:
* non-catalytic thermo-chemical reactor,
* refrigerant pre-cooling of fuel and refrigerant liquefiers, 
* refrigerant sub-cooling of photo-voltaic panels [10] to drive air 
* capture of motor vehicle draft to assist Cryo-GT compressor drive, 
* capture of structure induced draft to assist wind turbine-generator 
  drive, and
* cryo-capture of residual carbon dioxide. 

   This post describes the stationary and motor vehicle Cryo-GT prime movers and the infra-structure for supplying liquefied fuel and sink refrigerant. The reference vehicle and distributed generation fuels are liquefied methane and syngas, a mixture of carbon monoxide and hydrogen, synthesized from universally available organic materials. The reference refrigerant is liquefied air, which is readily condensed using recovered energy from renewable sources including wind, gasifier pressure and solar. Typical household electric, fuel and liquid air consumption are 35 kWh/d, 1.6 kg (3.5 lb) liquid methane/d and 13.6 kg (30 lb) liquid air/d, respectively. Cryo-GT performance is exemplified by 45 % compact vehicle (exclusive of recovered draft and deceleration), and 70 % station thermal efficiency. The Cryo-GT ranges in capacity from about 10 KWe for small vehicles to 1 MWe for a distributed electric station. Gasifier performance, exemplified by 70 % fuel conversion efficiency, requires estimated 18 kg/d (40 lb/d) of sun dried wood to supply household requirements above. Water gas shift and methanation reactions downstream of the gasifier reduce carbon dioxide emissions to 10 - 15 % and to 15 % - 25 % as with normal wood and coal combustion, respectively.

Air Blown Gasifier

   Recovered heat from the air blown gasifier powers a thermo-chemical reactor, which may operate on selected cycles in the gasifier temperature range; including Hybrid Sulfur (Westinghouse), Sulfur-Iodine [11] and Manganese Oxide [12]. The suggested reference cycle is the Hybrid Sulfur cycle, which is under advanced development by Savannah River National Laboratory and others. Gasifier outlet temperature of ~ 1100oC (2010 oF) at ~ 60 bar is well above required sulfur decomposition temperature of ~ 850 oC (1560 oF) and gasifier waste heat of ~ 40 % delivers hydrogen heating potential equivalent to gasifier syngas output. The thermo-chemical reactor replaces the steam evaporator of a Rankine cycle engine, such as used in commercially available Mitsubishi (MHI) gasifiers, to recover heat of recycled char. Recycling the char above slag liquefaction temperature of oC (~ 2200 oF) delivers syngas with exemplary H2/CO normal volume ratio of 0.33 (coal) and 0.67 (wood). Downstream of the gasifier, water cooled syngas (H2, CO and N2), expands through a syngas liquefier to provide liquefied gas with dissolved H2. The liquefied gases vaporize during cooling of Cryo-GT and air liquefier heat sinks, followed by separation of H2fromsyngas, water gas shift of CO to CO2, methanation of H2, and CH4fueling of both station and motor vehicle Cryo-GT’s. The methanation process uses dry ice deposed from the sub-ambient Cryo-GT sink, while minimal CO2is discharged to atmosphere.
   Illustrative performance is based on reduced system capacity to supply 35 kWh household demand from the Cryo-GT generator, requiring 240,000 Btu Cryo-GT fuel combustion at 50 %. With wood feedstock, a typicalsyngas mixture (mol fraction) from the gasifieris 51 % N2,27 % CO, 14 % H2, 5 % CO2, 3 % CH4,whichequates to 13.6 kg (30 lb) N2,7.2kg (16 lb) CO, 0.3 kg (0.6 lb) H2, 1.9 kg (4.2 lb) CO2, 0.4kg (0.9 lb) CH4.Water gas shift and methanation then yield 10.5 lb CH4lower CH4heating value = (5600 Btu/lb). After combustion in Cryo-GT 26 lbCO2to methanator and 8 lb to atmosphere this is 13 % of CO2from normal wood burning.
   Other advantages of “third way primary heat source” are: 
* support of gasifier combustion by the oxygen by-product of the thermo-chemical 
* additional heat recovery from gasifier slag discharge, and
* expanded application of Integrated Gasification Combined Cycle   
   (IGCC) plants, some of which are idled or converted to natural gas  
   due to high carbon emissions. 

Stationary Fuel Liquefier
   Syngas liquefaction from the air blown gasifier is by a proposed two-stage process. The first stage is water cooling of syngas from the gasifier at constant pressure to ambient temperature before sensible cooling by turbine expansion. The second stage is magneto-caloric heat lift of latent heat to a dry ice heat sink at –80 oC (-110 oF), which requires recirculation of vaporized syngas through the magneto-caloric device. The liquefied product is CO and N2with dissolved H2. A similar two-stage process may be optionally employed to liquefy the O2by-product of the thermo-chemical reactor, as required. 
   Dry ice for the magneto-caloric sink is available from Cryo-GT exhaust and from external sources. Power to the from recovered syngas heat downstream of the gasifier via a steam turbine-generator.
   The reference design point is based on liquefying the above described syngas mixture from the gasifier of 1 % H2,31 % CO, 2 % CH4,8 % CO2, 58 % N2(mass fraction). Estimated temperature of the liquid is –193 oC (-315 oF) and power from the steam turbine-generator to the syngas liquefier is about one-fourth as with a magneto-caloric liquefier providing normal heat lift to ambient atmosphere.

Stationary Cryo-GT Prime Mover

   The stationary Cryo-GT operates with “source + sink” storage”, in which sub-cooling of working fluid entering the compressor is by liquefied syngas prior to water gas shift and methanation. Sub-cooling is via a cryo-heat exchanger, Thermodynamic engine efficiency is a function of the temperature difference between source and sink relative to the temperature of either. Heat sink energy is stored in a refrigerant, just as heat source energy is stored in fuel. Storage density is approximately 16 times as compared to a Li-ion battery. Source temperature may range from ambient atmosphere in a fuel-less liquid air engine to approximately 890 oC (1630 oF) with sink temperature at -190 oC (-315 oF). The gas turbine is the reference engine for source plus sink storage because of the simplification afforded by external compression and convertibility of available micro-turbines and turbo-chargers to cryo -compression. Optional features to reduce sink refrigerant use are turbine blade cooling and quasi-isothermal turbine expansion.
   The reference design point is based on a 1 MW distributed generation output. Gas turbine efficiency is 70 % at 25,000 rpm with turbine compression ratio of 4; turbine inlet gas temperature of 894 oC (1640 oF); air compressor inlet temperature of –175 oC (-280 oF) and recuperator effectiveness of 90 %. Under these conditions methane consumption is 100 kg/h (220 lb/h). Gas turbine compression work is about one-fourth, as with ambient air intake. Emissions are reduced in proportion to fuel consumption and dry ice may be deposed from engine exhaust for other sink pre-cooling.

Stationary Air Liquefier

   The stationary air liquefier provides liquid air refrigerant for heat sink cooling of motor vehicle Cryo-GT prime movers. Liquefaction is by a vapor-compression machine, in which a cryo-compressor recirculates air and atmospheric make-up air sub-cooled by the syngas in a cryo-cooler arranged in parallel with the heat sink of the stationary Cryo-GT. The sub-cooled air then discharges through a two phase turbine expander into a liquid air separator, from which a liquid air product portion is drawn-off to an air dewar.

   Power to the air liquefier is available from recovered gasifier or external renewable sources and temperature of the liquid air is –193 oC (-315 oF). Required power is about one-third as with a vapor compression machine without sub-cooling and associated cryo-compression.

More detailed description of network components [Later]:
Shift Reactor/Methanator
Motor Vehicle Cryo-GT Prime Mover

Motor Vehicle Cryo-GT Liquefier

Advanced Solar PV Drive

Advanced Wind Drive
1. National Energy Technology Laboratory, “Entrained Flow Gasifiers,    
    MHI”, 2014
2. Kaufman, J.S., “U.S. Patents 7,398,841, 7,854,278”, 2008, 2010. 
3. Gorensek, M. et-al, “Solar Driven Thermo-Electrochemical Hybrid 
    Sulfur Process for Hydrogen Production”, AIMES 2018
4. Kleppe, J. and Schneider, R., "A Nitrogen Economy", ASEE, 1974
5. Kishimoto, K. et-al, “Development of Generator of Liquid Air 
    Storage Energy System”, Mitsubishi Tech. Review Vol. 35-3, 1998
6. Ordonez, C.,“Liquid Nitrogen Fueled, Closed Brayton Cycle 
    Cryogenic Heat Engine”, Energy Conversion and Management 41, 
7. Knowlen, C. et al, "High Efficiency Energy Conversion Systems for 
8. Center for Low Carbon Futures, “Liquid Air in the Energy and 
    Transport Systems”, ISBN: 978-0-9575872-2-9, 2013
9. Gabbar, H. et-al, “Conceptual Design and Energy Analysis of 
    Integrated Combined Cycle Gasification System”, Sustainability 
    2017,9,1474, 2017
10. Liebert, C. et-al, “Solar-Cell Performance at Low Temperatures & 
      Simulated Solar Intensities”, NASA 1969
11. Perret, R., “Solar Thermochemicla Hydrogen Production 
      Research”, SAND2011- 3622, 2011 
12. Rao, C. et-al, “Solar Thermochemical Splitting of Water to 
      Generate Hydrogen”, PNAS 114(51), 2018